1. Field of the Invention
The present invention relates broadly to infrared sensors. More particularly, the present invention relates to passive infrared sensors for typical use in motion detectors which automatically turn on lights in the presence of a warm body.
2. State of the Art
Infrared sensors for use in motion detectors are well known in the art of security systems and are often associated with security lighting. U.S. Pat. No. 3,958,118 to Schwarz and U.S. Pat. No. 5,066,855 to Lee describe passive infrared motion detectors which turn lights on in response to the approach of a warm body such as a person or a vehicle. In order to increase the sensitivity of these systems, it is known to provide an optical system in front of the infrared sensor. Such optical systems generally broaden the field of view of the sensor so that a warm body approaching the detector is detected from several angles. The typical range of a motion detector with an optical system is about fifty to seventy feet with an angle of view of about seventy-five to one hundred eighty degrees.
The detectors of the prior art are typically placed at the periphery of an area to be secured, e.g. a home, an office building, etc., with the sensor's field of view directed outward from the area to be secured. The lights turned ON by the detector are most often coupled to a timer which keeps the lights ON for a predetermined amount of time after the last motion was detected by the sensor. This type of lighting system is usually referred to as automatic, responding (turning lights ON) only when a warm body approaches the secured area from the monitored front of the detector system. Upon exiting the secured area, however, the problem is that the lights are not turned ON until the warm body passes the periphery of the secured area into the forward facing field of view of the sensor. Thus, the warm body (person) is exposed to the hazards of darkness for a length of time before being detected by the detector.
It is also known in the art, as described in the aforementioned patent to Lee, as well as in U.S. Pat. No. 4,644,147 to Zublin to use reflective surfaces to broaden the field of vision of a sensor. These disclosures, however, neither recognize the advantages of, nor provide a means for sensing the warm body from behind the detector so that illumination can be provided upon exiting an area.